The present invention relates to devices for improving failure mode performance of a braking system and in particular to a switch responsive to a braking system failure for disabling the system's proportioning valve or valves to thereby increase braking effectiveness of non-failed portions of the system.
Proportioning valves are now an established and well known component of braking systems for motor vehicles. These valves function to proportion the pressure of braking fluid from a source such as a master cylinder or booster between the front and rear wheels of the vehicle. Reducing the pressure to the rear wheels with respect to the front wheels reduces the tendency of the rear wheels to lock. Such valves may function with a fixed proportioning ratio or may, in more sophisticated variations, include height, inertia or load sensing means to further vary or proportion the application of pressurized fluid based upon loading of the vehicle. Such devices provide significant improvements in operation of the braking system under normal operating conditions. However, in the event of a failure in one of the circuits of a braking system, and in particular such a failure affecting one or both of the front wheels of the vehicle, emergency braking of the vehicle must be accomplished through the vehicle's operational hydraulic circuit. Under these half system conditions, the proportioning valve can reduce the effectiveness of the braking system by reducing the available fluid pressure at the vehicle's rear wheels.
Accordingly, there exists a need to provide a device which will respond to a braking system failure affecting the front wheels of a vehicle to disable or otherwise modify operation of the proportioning valve to increase the available braking pressure at the vehicle's rear wheels.
In its broader aspects, the invention is a failure compensating switch for use in a vehicle braking system which includes a plurality of braking circuits and at least one brake pressure proportioning valve for reducing the pressure of braking fluid applied to the rearwardly disposed wheels of the vehicle in proportion to a predetermined parameter such as braking pressure or vehicle wheel loading. The failure compensating switch comprises a differential pressure valve means connected between the braking circuits for operation between at least a first and a second operating condition in response to a pressure failure in one of the braking circuits. A disabling means is connected to the differential pressure valve means and to the vehicle's source of pressurized braking fluid for operation in response to operation of the differential pressure valve means to render the proportioning valve partially or fully inoperative.
In a specific embodiment of the invention, for use in a braking system of the cross-split variety, the differential pressure valve is interposed between the right front-left rear and the left front-right rear braking circuits. The differential pressure switch is normally in a neutral position and is bidirectionally operable in response to a braking system failure in one of the two braking circuits. The disabling means includes a piston device isolated from the braking system when the differential pressure valve is in its neutral position. The disabling means is fluidly connected to a source of braking fluid in response to operation of the differential pressure valve, and exerts a force against the proportioning valve to render same inoperative under these conditions.
It is therefore an object of the invention to provide a device for rendering a proportioning valve of a braking system at least partially inoperative in response to a braking system failure.
It is another object of the invention to provide such a device which includes a differential pressure switch for sensing a failure in one of the braking circuits of a motor vehicle and rendering a proportioning valve associated with a nonfailed circuit inoperative in response thereto.
Still another object of the invention is to provide such a device which includes a fluid-operated piston assembly which generates a force against the vehicle's proportioning valve in response to a failure of the braking system affecting the front wheel brakes of the vehicle to render the proportioning valve inoperative.
Another object of the invention is to provide such a device which will automatically return to an inoperative state wherein it has no effect on the vehicle's normal braking system when a braking system failure is corrected.
Yet another object of the invention is to provide such a device which is useable on a variety of braking systems including cross-split braking systems and systems incorporating anti-lock control devices.